"In the year of Christ 1666, on 2 September, at a distance eastward of this place of 202 ft, which is the height of this column, a fire broke out in the dead of night which, the wind blowing, devoured even distant buildings, and rushed devastating through every quarter with astonishing swiftness and noise…" (PROSE: Matrix)

As the Terileptil involvement was equally unknown, it was widely believed that the people of London were so scared they set fire to London. Alistair Gryffen noted this as an example of events concerning the spread of fear. (TV: Fear Itself)

The date of 2 September is derived from prose sources. It's Matrix and the novelisation of The Visitation, not the serial, that give the specific date. The novelisation's rendition of the scene in episode one where the Doctor and Nyssa are sneaking around the manor house contains a line where he claims it's the first of September. Since night falls during the course of The Visitation, it's reasonable to assume that the Great Fire occurs on 2 September. Matrix takes away all guesswork by quoting a commemorative plaque which flatly gives the date.